Hückel and Möbius aromaticity and trimerous transition state behaviour in the pericyclic reactions of [10], [14], [16] and [18]annulenes †

The pericyclic reactions of a series of [n]annulenes (n = 10, 14, 16, 18) reveal unusual differences in their behaviour related to their aromaticity. For n = 10, 14, synchronous pericyclic bond formation (trimerous behaviour) is possible, whereas the synchronicity is not apparent for n = 18. The formally anti-aromatic system n = 16 exhibits valence bond isomerism, asynchronous pericyclisation and a novel conformation which appears to be an example of a Heilbronner Möbius aromatic, as indicated by nucleus independent chemical shift calculations.